1. Field of the Invention
The invention relates to a combustion apparatus and method for burning fuel in a mixture with air with the aim of producing hot gas for various applications. More specifically, the invention relates to a combustion apparatus and method using a combustor with recirculation flow. The invention further relates to an apparatus and method for igniting and burning a mixture of fuel and air. A combustor of this type may be used for burning lean and super-lean fuel and air mixtures for use in gas turbine engines, jet and rocket engines and thermal plants such as boilers, heat exchanges plants, chemical reactors, and the like. The apparatus and methods of the invention may also be operated under conditions that favor fuel reformation rather than combustion, where such a reaction is desired.
2. Description of Related Art
(The following description or related art should be read in light of the definitions of certain terms provided in the detailed description below.)
In a typical combustor, combustion air and fuel (which may or may not be premixed) is introduced through an inlet opening to a combustion space, where the combustion process occurs. Recirculation flow may be present, in which the burning gases are recirculated within the combustor before rejoining the main combustion flow. Introducing a high-speed, high-temperature, large mass recirculation flow injects thermal and kinetic energy into the main combustion flow, thus allowing stable combustion of lean and very lean fuel/air mixtures, and lowering harmful emissions, among other advantages.
Although a recirculation flow is present in many combustion methods and apparatuses, recirculation flow in existing combustors occurs within the combustion space without being confined to a special space for an organized movement. As a result, existing combustors do not maximize the velocity of the recirculation flow, and thus do not maximize the amount of thermal and kinetic energy injected into the main combustion flow, which would be desirable for efficient and reliable combustion of lean and very lean fuel/air mixtures.
For example, U.S. Pat. No. 4,586,328 to Howald discloses a generally toroidal-shaped combustor in which the combustion mixture burns along a generally toroidal-helical gas flow path. However, the recirculation flow (burning gas) that is fed back to the inlet opening zone within the combustion chamber does not have a velocity that is high enough; hence very low energy is supplied to the fresh fuel/air mixture. The outlet of the periphery of the torroidal flow path is into the turbine. Further, in Howald, additional cooling flows are introduced between the air flow and the flow of recirculated burning gas. Consequently, the conditions for injecting the burning gases into the air flow or into the fuel/air mixture flow are impaired, and the amount of energy supplied by the recirculation flow to the fuel/air mixture is low. The solution is to make the fuel/air mixture richer, which is not desirable because it results in a higher combustion temperature, incomplete combustion, and increased harmful emissions.
U.S. Pat. No. 3,309,866 to Kydd discloses a process and apparatus for flameless gas combustion in which recirculation occurs (i.e. hot, substantially completely burned gas within the combustor is combined with the fuel/air mixture entering the combustor). Like Howald, the combustor disclosed by Kydd does not maximize the velocity of the recirculation flow, thus resulting in a low level of energy being supplied to the main combustion flow. As in Howald, the flow along the periphery of the torroidal circulation area also feeds into the turbine. In addition, the combustor in Kydd includes a baffle in the form of an annular plate with holes, so the burning gases do not directly flow into the fresh fuel/air mixture, thereby impairing the conditions for injecting the burning gases into the fuel mixture. The main disadvantage here is thorough mixing, with the fuel and air mix admitted and thoroughly mixed with almost completely burned gases that are in a swirl motion.
In U.S. Pat. No. 5,857,339 to Roquemore et al., a trapped vortex combustor with hot gas recirculation to the main flow inlet has fuel and air inlets for admitting fuel and/or air to the recirculated hot gases before the hot gases meet the main flow. Similarly to other known combustors, the temperature of the recirculated hot gases meeting the fresh fuel and air mixture decreases rapidly because, among other things, of intensive fuel reforming processes which are occurring in the fresh fuel and air mixture. In this case, adding air and/or fuel to the recirculated hot gases is counterproductive because the temperature of the recirculated hot gases will be already lowered before they meet the main flow. The geometry of the combustion space is such that the recirculated hot gases meet the main flow as close as possible to a co-current flow. This means that the primary objective is to achieve the lowest hydraulic losses possible when the recirculated flow meets the incoming main flow. This geometry of mixing of the two flows is very disadvantageous, because the “mild” conditions at collision of the two flows result in a very poor energy transfer between the flows, and non-uniformity or temperatures at the main flow inlet can reach up to 100%, and the inner layers of the incoming main flow may not be heated at all. This results in poor heating of the incoming main flow with the resulting flameout. A typical temperature profile for combustors of this type (see FIG. 19) shows that the temperature of the incoming main flow in a trapped vortex combustor at the inlet to the combustion space remains practically the same as the temperature of the main flow fed to the combustor. The consequence of this is high non-uniformity of combustion temperature axially along, and radially of the combustor, which translates into lower flame stability when the fuel and air mixture becomes leaner and also to high CO and NOx emissions. It should be added that the use of additional air and/or fuel inlets in the path of the recirculation flow is very disadvantageous because they create non-uniformity of the velocity profile within the recirculation flow, which translates into increased non-uniformity of energy transfer between the recirculated hot gases and the incoming main flow.
In U.S. Pat. No. 6,295,801 to Burrus et al., a combustor uses the trapped vortex operation principle to sustain a pilot flame. This design has the same disadvantages as those described above. The main advantage of this trapped vortex design here is stability of the pilot flame. This is done because the main flame stability could not be achieved in the prior art without using additional devices. The vortex velocity cannot be equal to the inlet flow velocity. Air is fed to the vortex zone through ports having a velocity coefficient of about 0.75. The main air flow is admitted to the combustor through profiled passages having a velocity coefficient of about 0.9. With an ideal isentropic velocity of 100 m/s, the main air flow velocity will be 90 m/s, and the vortex velocity will be 75 m/s. The velocity the flow fed to the vortex could be increased with the available pressure differential before feeding air to the vortex, or the pressure differential can be increased. It should be noted, however, that the temperature of the fluid admitted to the vortex should not be below the gas temperature in the vortex, i.e., the combustion products should be added to the vortex. The main flow undergoes sudden expansion, which results in a velocity decrease. In general, the turbulent character of vortex flow results in a velocity decrease. All these factors do not allow additional energy to be supplied to the incoming main flow.
It can be summarized that the use of trapped vortex in combustors in the prior art is mainly characterized by heating the surface layers of the incoming main flow, which in itself is not bad and can bring about certain improvements in sustaining lean mixture flame. On the other hand, the superficial heating cannot result in any dramatic improvement of flame stability and emission reduction.
In these prior art recirculation flow combustors, the recirculation flow of hot gases is diluted (cooled) with a flow of secondary air and then the cooled recirculated gases are directed to the primary air the inlet, which should be heated. (See FIG. 20.) Fuel is added to the hot recirculated gases diluted with the secondary air flow before it meets the primary (main) air flow. Admitting fuel to the hot recirculated gas results in a very non-uniform conditions for combustion because a very small quantity of fuel cannot be mixed thoroughly with a very large quantity of the recirculated gases and secondary air. Fuel reforming will be very intense and non-uniform in this case with the ensuing cooling. The fuel is then ignited, and the temperature of the gases increases, but this increase will be partly used to compensate for the temperature reduction because of the fuel reforming. The flow then meets the primary (main) air flow (which is actually a secondary flow because the mixture is already burning) and is again cooled. The main flow cannot be heated at the inlet because the recirculated hot gases have been already cooled down twice (first, with the secondary air flow and second, by admitting fuel), and the recirculation flow heating by fuel burning has been partly spent to compensate for reforming temperature losses. It is not possible to heat the main flow at the inlet uniformly over the entire cross-section because the result depends entirely on the turbulent mixing of the two flows, which cannot assure uniform mixing through the entire volume. This reliance on the turbulent (mechanical mixing) is all the more questionable because the two flows move practically co-currently.
The temperature in the recirculation flow in all the above-described combustors cannot be higher than the TIT (turbine inlet temperature). (See FIG. 21.) The preferred temperature in the recirculation flow based on NOx and CO emissions tradeoff is 1100–1200° C. Adding air and/or fuel to the recirculated hot gases results in a reduction in the recirculation gas temperature. There are two major consequence of this. First, CO emissions will increase. Second, more combustion products will have to be added to the incoming flow in order to increase the incoming flow temperature, which causes an increase in fuel reforming, thus bringing temperature down. Therefore, the use of trapped vortex and recirculated flow in the prior art combustors, while bringing about certain improvement in flame stability and emission performance, has not been able to result in any breakthrough.
U.S. Pat. No. 5,266,024 to Anderson discloses the use of a thermal nozzle to increase the kinetic energy of a flow of oxidant to a blow torch by supplying heat to the flow.
U.S. Pat. No. 1,952,281 to Ranque discloses the phenomenon, and apparatus for creating the phenomenon, whereby in a vortex tube having one tangential inlet flow of compressed fluid, heat is transferred between rotating layers of fluid in the vortex tube, resulting in a separation of the rotating fluid into a hot outer flow and a cold inner flow, which may be taken from separate outputs.